1. Technical Field
The present invention relates generally to turbines and more particularly to a turbine disk having an enclosed friction damper.
2. Background Art
Turbine disks and blades are commonly subject to high cycle fatigue failures due to high alternating stresses as a result of resonant vibration and/or fluid-structural coupled instabilities. Turbine disks are typically designed to avoid standing wave diametrical mode critical speeds within the operating speed range. High dynamic response occurs when the backward traveling diametrical mode frequency is equal to the forward speed diameteral frequency which results in a standing wave form with respect to a stationary asymmetric force field. Turbine blades are designed to avoid having any of the blade natural frequencies from being excited by the stationary nozzle forcing frequencies in the operating speed range.
In conventional turbine wheel assemblies, conventional blade dampening techniques are typically employed to reduce the fluid-structure instabilities that results from the aerodynamic forces and structural deflections. Accordingly, it is common practice to control both blade and disk vibration in the gas turbine and rocket engine industry by placing dampers between the platforms or shrouds of individual dovetail or fir tree anchored blades. Such blade dampers are designed to control vibration through a non-linear friction force during relative motion of adjacent blades due to tangential, axial or torsional vibration modes. Blade dampers, in addition to the blade attachments, also provide friction dampening during vibration in disc diametral modes.
Integrally bladed turbine disks (blisks) are becoming increasingly common in the propellant turbopumps of liquid fueled rocket engines and gas turbines. While the elimination of separate turbine blades reduces fabrication costs, the monolithic construction of integrally bladed turbine disks eliminates the beneficial vibration damping inherent in the separately bladed disk construction. Accordingly, the above-mentioned damping mechanism is not feasible for integrally bladed turbine disks unless radial slots are machined into the disk between each blade to introduce flexibility to the blade shank. The added complexity of the slots would increase the rim load on the turbine blade and defeat some of the cost, speed and weight benefits of the blisk. Consequently, the lack of a blade attachment interface results in a significant reduction in damping and can result in fluid-structure instabilities at speeds much lower than the disk critical speed and at minor blade resonances.
Rim dampers have been utilized by the gear industry to dampen diametral modes of vibration in thinly webbed large diameter gears. In such applications a split ring or series of spiral rings are preloaded in one or more retainer grooves on the underside of the gear rim. At relatively low rim speeds the centrifugal force on the damper ring provides damping due to relative motion when the gear rim experiences vibration in a diametral mode. This method of friction damping, however, is not feasible at high rim speeds because the centrifugal force on the damper ring is of sufficient magnitude to cause the damper to lock-up against the rim. Lock-up occurs when the frictional forces become large enough to restrain relative motion at the interface, causing the damper ring to flex as an integral part of the rim.
It is a general object of the present invention to provide a cost effective yet robust dampening mechanism for conventionally-bladed and integrally bladed turbine disk assemblies.
It is another object of the present invention to provide a dampening mechanism for a turbine disk assembly which positions a damper member between two adjacent tip shrouds to absorb vibration.
It is a further object of the present invention to provide an integrally bladed turbine disk assembly having damper members which are encased between two adjacent tip shrouds.
In one preferred form, the present invention provides an integrally bladed turbine disk assembly having an integrally bladed turbine disk and a plurality of captured damper members. The integrally bladed turbine disk includes a plurality of radially outwardly extending turbine blades and a plurality of damper apertures. Each of the turbine blades terminates at its distal end at an integral tip shroud. Each of the damper apertures is formed between two adjacent integral tip shrouds and includes a first slotted portion and a second slotted portion. The first slotted portion is formed concentric to the two adjacent integral tip shrouds, extending circumferentially and axially through the two adjacent integral tip shrouds. The second slotted portion extends radially outwardly between the two adjacent integral tip shrouds. The damper member is disposed in the first slotted portion of the at least one damper aperture, being preferably encased in the two adjacent integral tip shrouds when the integrally bladed disk is formed. The damper is frictionally engagable with at least one surface of each of the two adjacent tip shrouds to dissipate energy when relative motion occurs between the two adjacent integral tip shrouds to dampen vibration. The normal force on the interface between the damper and the two adjacent integral tip shrouds is a function of the centrifugal force acting on the damper mass due to the rotational speed of the integrally bladed turbine disk.